Many transmission systems utilize an optical fiber network of interconnected optical fiber cables to enable optical communications between network locations. Typically, an optical fiber cable consists of a plurality of optical fibers surrounded by a protective sheath. Individual optical fibers consist of a small diameter core of low-loss material such as glass or plastic surrounded by a protective cladding that has a slightly lower index of refraction than the core. As is well known, a fiber optic cable operates to guide transmitted light pulses over distances with small pulse attenuation (i.e., low signal loss).
Due to the fragile nature of today's commercial fiber, there is a need to protect the fiber from external sources of stress, such as bending, pressure and strain, which can damage the fiber and/or cause degradation of the signal being transmitted via the fiber. For example, a fiber should not be bent sharply anywhere along its path. In addition to the possibility of breakage or fracture, if a fiber is bent past a critical angle, portions of transmitted light pulses will leak out, rather than being reflected within the fiber core, thereby attenuating the transmitted light pulses and degrading signal quality. Accordingly, it is necessary that a fiber be routed so that bends in the fiber be of a sufficient radius to substantially avoid occurrence of such light leakage.
The radius below which a fiber should not be bent to avoid light ray leakage is characterized as the minimum bend radius. Typically, the minimum bend radius varies with fiber design. However, in all fiber designs, bending the fiber with a radius smaller than its minimum bend radius may result in increased signal attenuation and/or a broken fiber.
Ordinarily, a unique fiber routing will be required to transmit light pulses between network locations. Over this unique route, light pulses may be propagated across several different fibers. At each transition from one fiber to another, individual fibers are connected, thereby enabling light pulses to be carried between a first fiber and a second fiber. In many cases, such as at a central office for the communications system, large numbers of fiber connections must be made and a fiber administration system is employed to manage the various connections.
In many fiber administration systems, as the optical fibers in a network enter the central office, they are directed into a fiber distribution frame where the individual optical fibers are terminated in an organized manner. Such fiber administration systems are exemplified by the LGX® Fiber Administration System that is currently manufactured by Lucent Technologies of Murray Hill, N.J., the assignee herein.
A fiber distribution frame typically provides an equipment rack housing a column of spaced apart shelves and/or horizontally slideable drawers for distributing a large number of fibers. Each shelf and/or drawer has an enclosed chamber with an aperture through which fiber cabling may pass. Fiber connections are fashioned within the enclosed chamber of the shelf. In a typical application, the frame serves as an interface between central office switching equipment and a network of transmission lines terminated at the central office.
Pre-terminated fiber shelves provide a convenient means of installing fiber cable connections at a network location. Such shelves are equipped with optical connectors that have been assembled onto the end of a fiber cable stub (i.e., a short length of fiber cable). Although fiber cable specifications normally require that the fiber cable not be bent in diameters smaller than 20× the cable diameter, if the fiber cable sheath is removed, individual fibers can be bent, or coiled, to smaller diameters. This permits the routing of a given number of fiber cables in a smaller area of a distribution frame. For this reason, the end of the fiber cable stub, which enters the shelf, are typically unsheathed. The fiber connections for a pre-terminated shelf are also fashioned during manufacture of the shelf, so that no field connectorization is required during installation.
Because of the delicate nature of unsheathed fibers and the need to maintain fiber bend limits for fibers transitioning to connectors in a shelf, the stub must be securely fastened to the shelf during manufacture. The stub may be fastened to provide either a top entry direction or a bottom entry direction. After manufacture, it is not desirable to loosen the stub from the shelf or to attempt to reconfigure the entry direction of the stub, for example, during installation of a shelf. Because the stiffness of fiber cable and the bend radius constraints associated with fibers, such reconfiguration attempts are prone to craft errors that could significantly damage the stub and/or fibers. At best, special care is required to reconfigure stub entry direction without exceeding fiber bend limits or breaking fibers.
Often, the specific fiber cable routing for a network location is not determined prior to set-up of the location. Consequently, the desired stub entry direction for shelves to be installed at the location is also not known. For this reason, and for manufacturing efficiency, shelves are typically manufactured as configured only for a top stub entry direction. Thus, shelves configured for top stub entry are shipped to a network location regardless of the location requirement. This causes significant problems when the network location requires a shelf configured for bottom stub entry.
Since stub entry direction is not easily reconfigured, a typical work-around for altering stub entry direction is to route the stubs through a circuitous route up the opposite side of the distribution frame, and then to loop the stubs over the top of the frame back down the other side. However, this method requires an extra length of fiber cable for the circuitous loop. In a typical application where multiple shelves are installed in one frame, this circuitous looping is likely to result in cable congestion—there being twice as much fiber cable in the vertical area of the frame as there would be if the fiber cable route were restricted to a single side of the frame. Furthermore, the excess fiber cable may block the path of other fiber cables that are routed through the top of the frame. In addition, the stiffness of fiber cable makes it difficult to manage fiber cable loops within the width of the frame. Therefore, this work-around is only practical for use with cables that allow a tight bend radius.